A thermofilter can be built using easily obtainable materials and usual household/shop tools. A single basket is necessary. A double basket will not work well because there will not be adequate room between the thermocouple exit of the basket and the portafilter sidewall. A K-type thermocouple is used and a reader is needed. You may use other appropriate thermocouples or sensors.
The first step is to seal the bottom of the filter basket with a thin layer of JB Weld, leaving only a hole which will allow a flow of approximately 50 ml/25 seconds.
Starting with a single basket, a wire is used to keep a small opening in the bottom of the basket. The single hole will approximate puck flow of about 50 mL per 25 seconds. This is with a 0.011 inch wire. This size is probably the easiest wire to use and is available at guitar string outlets. Smaller wires are available but more difficult to work with.
Prior to insertion, coat the wire liberally with silicone lubricant spray and allowed to dry so epoxy used in the basket will not adhere to the wire and allow the wire to be pulled back out. The wire should be inserted through a hole in the center of the bottom of the basket and taped in place so that it is vertical. About 1/16 to 1/8 inch of JB Weld epoxy should be layered in the bottom of the single basket and will hold the wire in place. It should look something like this photo.
When the epoxy is fully hardened pull the wire, coat with more silicone and reinsert. The next step is to place a short piece of simple plastic drinking straw around the wire. This will become the pathway for brew water to exit the hole.
It should look about like this after the first JB Weld mix. You can use other epoxy, but High Temperature epoxy only. Many epoxy gets soft before the 200F approximate brew temperature.
JB Weld is readily available. Let it fully set/harden, probably overnight. You can get some leveling of the JBW if you warm it until it thins a bit. Leveling is not so important at this step, but heating will also slightly accelerate the curing process, and you will want to get a feeling of heating so that you can level the last batch of JBW for the top of the puck.
When JBW is fully hardened, pull the wire out the bottom. You need to be able to re-insert the wire if a coffee ground plugs later, so get a feel for it now. It will probably take pliers to grip for pulling the first time. Re-silicone and re-insert and pull and insert a few times until comfortable. It you cut the wire on the end that goes through the epoxy you may have trouble. Wire cutters slightly crush and flatten the wire. If you have to use a cut end, deburr it or remove the crush so that it is round. I held the end of the wire against a Dremel sanding drum or grinder and turned the tip over, sharpening it if you will.
As for that wire, two things:
I made a loop at the bottom end after discovering in a previous attempt how very hard it can be to pull out. Then I used needle nose pliers and wound the loop and the wire around them before pulling. Also I had applied silicone a couple of times and sharpened the end of the wire. It worked. In the pictures you can see that the basket is supported on the rim of a short glass. The loop of the wire and a bit of blue tape are supporting the wire so it doesn't just fall back out. It took a bit of fiddling to find the right depth, but if you just put the basket into the vise the wire will ...slip slide away...
You can also tape to the bottom of the basket and then twist the tape, corner of the tape, 90 degrees and
tape to the upright wire.
The following photo shows hole placement. The ink dot is where you don't want the hole. The actual drilled hole could probably be anywhere between the hole shown and below the dot. You can place the hole where you think it will be most easy to drill, yet not on the sidewall that will be parallel to PF wall where there will not be room. That hole was drilled with a 3/32 inch fresh sharp drill. You may need to make a slight indent to get the drill started. A vise is strongly suggested for holding the basket.
Here is what can happen if you use a double basket. A double basket could probably be used if the wire was fixed in the center with the first mix of JB weld. Next the thermocouple hole on a double basket could be drilled from the bottom, out toward the sidewall. The thermocouple could then be fitted from the bottom into an arc into the central straw similar to what is done with a single basket.
That one looked really good until I put it into the portafilter and watched all of the water come up
around the sides. The exiting wire caused just enough of a disruption in the seal to make the whole thing useless...
Back to the drawing board and started over with the single basket...
Here's another photo showing the deburr tool, though others may work. There should not be sharp edges at the basket edge. Toothpick shows the approximate thermocouple path and elevation at the "puck" center.
A Teflon coated K thermocouple was used. They are inexpensive and fine for this application. Fiberglass insulation tends to fray and unravel and is a little less desirable to work with.
The height of the "puck" is fairly critical. It you make the puck low, you have too much sitting water and then not good thermofilter response. Too high and it will hit the screen and not work at all. You will want the thermocouple and bead at about the ridge height, or probably just below. Adjust the straw height and bead hole in the straw accordingly. Also, spread the wires a bit and form an "eye." The eye can lay horizontal and then the TC make a smooth arc to exit. Also note that there is the central 0.011 wire coming up out of the straw.
When all is taped and held in place, the final batch of JBW can be mixed and "poured." You can use a couple of drops of acetone to slightly thin it to help level. "Pour" is not really descriptive as you will pour, spoon, and scrape it in. The couple drops of acetone helps, not necessary if you don't have it, and a little heat will help also. You can heat a little to help pour, and then heat the basket to help level and smooth. Hint: heat gun and oven glove or similar.
The cleaned up final product.